TuKNKi{, Morplioloiiv of the Avian Brain. \ i^ 



This description gives the appearance of the tectum in 

 the caudad portion of either optic lobe. In the cephah^d 

 portion of each lobe, stratum twelve is much wider, and the 

 cells of thirteen have become intermingled with the fibres 

 that lie entad to the nidulus (Plate XVI, Fig. 2). leather, 

 in the caudad portion of each optic lobe the fibre tract that 

 forms the entad boundary of this nidulus lies immediately 

 ectad to two large niduli, while in the cephalad portion of 

 the same lobe that tract lies immediately ectad to the epithe- 

 lium of the mesencephalic ventricle. It may be of interest 

 to note that, in the specimens examined, this epithelium is 

 represented by two or more rows of densely packed nuclei. ( ') 



To recapitulate, in general, the avian tectum opticum 

 consists of three parts: 



I. Four dense concentric shells of Deiter's corpuscles. 



3. These are isolated from each other and from the re- 

 mainder of the tectum by five cell-less neuroglia layers. 



3. A nidulus of elongated cells. The general trend of 

 this nidulus is parallel to the concentric shells. 



I have been agreeably surprised to find that this arrange- 

 ment appears to be constant. Two apparent exceptions have 

 been noticed. In one case there were only three concentric 

 shells, and in another there was none. However, in each of 

 these cases it is probable that the specimens were in a patho- 

 logical condition, for in one case the brain had been used in 

 an extirpation experiment, while in the other the head con- 

 taining the brain spent a mid-winter's night upon the lloor 

 of a butcher's shop. If the number of these concentric shells 

 of the avian tectum opticum be constant, they must have a 

 special function to perform. It seems very suggestive that 

 Deiter's corpuscles are factors of much greater importance 

 than is usually admitted. 



Above I stated that entad to the tectum and caudad to 



I This peculiarity has been noticed by Stieda. After describing the nerve tract 

 mentioned above, he remarks: ''An diese reiht sich das I'flaster epithel des Vcntrikels, 

 dessen Kerne allein sichtbar sind." Op. cit., p. 44. 



